In the interest of energy conservation and the protection of the environment from both noise and objectionable combustion exhaust gases, the technical world is striving to develop methods and apparatus that will effect the most complete possible combustion of fuel with the best possible thermal efficiency and with the least possible noise emission.
As is known, burners for heating oil that are in the capacity range of one to 1000 kg. per hour are preponderantly of the type in which the liquid fuel is atomized under relatively high pressure to produce oil droplets that emerge from the nozzle in a more or less conical cloud. The droplets in this cloud are of various sizes, and there is no consistency in the distribution of the various size droplets across the cloud.
For good combustion of the atomized fuel it is of critical importance that the oil droplets be thoroughly mixed with combustion air and that the oxygen component of the air have the proper relationship to the atomized fuel with which it is intended to combine in the combustion process.
According to conventional practice, formation of the fuel-air mixture is initiated in a so-called mixing device and is continued and ended in the combustion chamber of the heat producer (furnace or the like). In general, mixing devices are so arranged that mixing of atomized fuel with combustion air begins directly after the fuel leaves the atomizer nozzle, in the plane of a so-called flame holder. The combustion air is conducted to the atomized fuel stream at a fairly high velocity and carries the fuel droplets along with it through an igniter and into a burner duct.
To some extent mixture formation and combustion occur sequentially in point of time, but considered across the whole volume of the fuel-air mixture, both in cross-section and in the stream flow direction, both processes are occurring simultaneously. Mixture formation and combustion therefore influence each other mutually and are determined to a substantial extent by the geometry of the combustion chamber and the thermodynamic relationships that develop in it.
Although the quality of the fuel-air mixture is to a large extent dependent upon the size of the oil droplets and the uniformity of their distribution in the spray issuing from the atomizer nozzle, commercial nozzles of like size and presumably like characteristics provide very different spectra of droplet size and distributions across the stream cross-section. This leads to unsatisfactory combustion and impels the use of counter-measures such as increasing the velocity of the combustion air stream, and/or increasing the excess of combustion air. However, this has undesirable secondary effects, including greater heat loss due to higher exhaust gas flow volume, the formation of peripheral fuel clouds at the flame root and the flame belly, the supercooling of the flame holder and its consequent substantial sooting, and increase of the noise level.
A wide variation in atomizing characteristics is particularly evident with pressure atomizer nozzles for oil throughputs of less than 2 kg/hr., by reason of their necessarily small bores and channels. The uncertain character of the output of such a nozzle is further aggravated by its sensitivity to sooting. In this capacity range, unsatisfactory oil cloud quality can be compensated for to a limited extent by increase of combustion air flow. But there are technical and economic limits to the permissible reduction of stream cross-section and the increase of excess air that accompany this expedient. Nevertheless, on the basis of experience, high combustion air flow velocities have been relied upon to provide good combustion qualities for conventional mixing devices with flame holders.
Thus, so-called combustion aids for improving combustion are known in combustion technology. These include whole or partial lining of the combustion chamber with refractory ceramics, and/or the provision of a duct of scale-free steel which wholly or partially surrounds the flame along its whole length or a part of its length.
Other such measures include lengthened burner ducts in various structural forms, with or without ceramic linings for heat storage and/or for supporting recirculation of combustion air. A limitation upon the employment of this last expedient is that the maker of the burner has no constructive influence upon the form of the combustion chambers in heat generators, particularly in the medium, small and very small capacity ranges.
The diameter of such a burner duct is, as a rule, so selected that no oil drops impinge upon its wall surface, anywhere around it or along its length, so that the flame can be conducted through the longest possible stretch. This leads to relatively large and long burner ducts, and consequently to substantial depths of immersion in the combustion chamber.
Measures for effecting a controlled recirculation of combustion gases are relatively expensive and create a need for devices to assist in starting the burner, such as motor controlled air dampers. These are necessary because the high stream velocity of the operating jet, required for stimulating recirculation, tends to prevent ignition at full load.
A large number of burner devices of this type have become known, differing in concept and operating according to various methods. Thus, there has been published a disclosure of a method of combusting liquid fuels wherein heating oil and deisel oil are burned with air by first atomizing the fuel with a mechanical atomizer, then tangentially introducing a further gaseous medium, after which air is introduced in a coaxial, same direction flow. This, like other burner apparatus, cannot be operated with a stoichiometric fuel-air relationship, so that soot is formed if the apparatus is operated with an insufficient excess of combustion air in an effort to maintain a near-stoichiometric relationship. The soot thus formed coats the combustion chamber and its outlet portions and very seriously deteriorates the heat transfer capabilities.
To avoid undesired soot formation, burner devices are as a rule operated with so much excess combustion air that--although the fuel is completely combusted--the flue gases are emitted with a substantial oxygen content. Optimum combustion efficiency cannot be achieved in this manner. Furthermore, experience shows that the very employment of excess air prevents the attainment of stoichiometric combustion, so that soot formation occurs notwithstanding.
A German Published Patent Application, DE-OS No. 2,511,500, proposes a method for burning liquid fuel which would purportedly permit operation without soot formation, under stoichiometric conditions and even with less than stoichiometric conditions, that is, with too small a proportion of combustion air. These objectives are said to have been realized with a variety of measures and structures by a stabilization of the mixture flow, accomplished by control of stream flow and stream temperature. Such control is said to be afforded by means of an insert downstream from the atomizing and mixing zone which produces a contraction of the stream flow that follows an expansion of it and is in turn followed by another expansion at the eye of the flame. With this it is also recommended that there be a markedly widened diffuser of ceramic material following the nozzle mouth; or else a ceramic combustion air duct of constant inner cross-section which is connected to its outlet and which is narrowed ahead of the atomizer nozzle, and then constant air introduction ducts of double-cone-shaped design with a cross-section that first widens, then narrows, or a cross-section that first narrows, then widens. Almost all forms that are possible with the basic arrangement are shown and described, so that for the combustion technician, and particularly for the atomizing technician, there is a disclosure of the entire possible field of combustion devices of this type, irrespective of how and why they can solve the problem and whether or not they can do so.
It is interesting that this disclosure points out that the portion which is downstream from the nozzle and which defines the mixing chamber should be of heat retaining material in order to stabilize the flame; and it specifies ceramic parts for this purpose. However, the significant teaching of this publication from the standpoint of the present invention is its advice that the configuration of the burner duct is to be so selected that combustion air is brought into the mixture stream of oil droplets and supplemental medium but that it must not in any event be so configured so as to permit oil droplets to settle on its convergent wall downstream from the burner.
Insofar as one adheres to the teaching of the publication that no oil droplets can settle on the narrowing wall, no amount of experimentation with the burner apparatus disclosed in it will lead to a solution of the problem to which it is supposedly addressed.
Basically, it can be said that the known embodiments of combustion devices, and the known methods of operating them, mainly relate to the capacity range above 2 kg/hr., and are unsatisfactory under intermittent operating conditions, particularly evident from relatively heavy soot formation during burner start-up.